Displaying
Pictures on Mechanical TV Sets

We use three techniques to display pictures on our
mechanical sets. First, we have a mechanical camera that generates 60 line
pictures for display on our RCA set. Visitors to the museum can see their
friends on this setup.

Finally, we use a PC to generate images for our Baird
Televisor (30 lines):

Using a PC to
Generate 30 and 60 Line Pictures

This is a technical description of how low definition video is
generated to allow the mechanical sets in our collection to work.

First, a video is produced using a standard camcorder. Care has to be
taken with the subject matter, because only closeups of faces, etc.
will show up on the low definition sets.

Then, the video is played back into a video capture card in a PC.
Snagit, a capture program, then converts the video to an .avi file.
For the 30 line Baird standard, 12.5 frames are captured per second.
For the 60 line standard, 20 frames are captured per second.

The .avi file is then converted to individual .bmp files, one per
frame, by a program called AVItoBMP.

A Visual Basic program is then used to read each .bmp file and
convert it to 30 or 60 line standard. The 30 line standard has
vertical scanning lines, while the 60 line standard has horizontal
lines. The video is recorded on the right channel. For
synchronization, sync pulses are recorded on the left channel (more
on this later).

This program then creates a .wav file which contains the data from
each frame sequentially. This .wav file is played by the PC using the
Media Player.

The right channel output of the sound card (video) in the PC is
connected to one channel of a 40 watt stereo amplifier which it turn
feeds the neon tube in the receiver. An interface circuit is
necessary to provide the DC bias for the neon and to increase the
voltage swing of the video to the neon.

Synchronization is different for the 30 line and 60 line systems. For
the 30 line system, a sine wave at the line rate (375 Hz) is placed
on the left channel. This wave is amplified by the other
channel of the stereo amplifier, which feeds the phonic coils on the receiver.

The 60 line receiver relies on a synchronous motor for
synchronization. Since it is difficult (if not impossible) to
synchronize the PC sound card output to the power line frequency used
by the synchronous motor, a 60 Hz sine wave (3 cycles per frame of
the video) is placed on the left channel to drive the motor.

The sine wave is then amplified by the second channel of the stereo
amplifier. The output of the amplifier is then fed to the motor
through an impedance matching transformer.